Whirlwinds in rust: antiferromagnetic vortices spotted for the first time

18 June 2018

Vortices are beautiful structures that are encountered in nature at all length-scales, from the nanometer to the billions of light years. In a paper appearing today in Nature Materials, the Oxide electronics group at the University of Oxford and their collaborators at University of Wisconsin-Madison (USA) and the Diamond Light Source (UK) describe how they used a synchrotron-based microscopy technique to image an unprecedented form of magnetic vortices in thin films of hematite (α-Fe2O3, a form of ordinary rust). This work was selected as the cover article for the July 2018 issue of Nature materials and promises several new avenues for exploitation in both fundamental and applied research.

In their α-Fe2O3 thin films, the authors observe a dense network of antiferromagnetic vortices and anti-vortices, regions where the magnetisation of the sample whirls around a central core. They show that this vortex network, while stable to small perturbations, can undergo mass vortex-anti-vortex annihilation by application of relatively larger magnetic fields. Of fundamental importance is that this vortex network represents a purely magnetic analogue of the Kibble-Zurek model, a model which describes cosmic string formation in the early universe. Hematite thus provides an opportunity to study the details of this model in a condensed matter system, easily controlled in a laboratory setting.

On the applied side, the authors show that their network of antiferromagnetic vortices can be directly imprinted onto an adjacent ferromagnetic layer (Co), forming a network of so-called ‘merons’ with a net moment at their core, similar to the ‘eye of the storm’ in a hurricane. These merons have the potential to be used in future devices, where the net core magnetisation can be used to store binary information.

Having observed this vortex network the next step for these researchers is to investigate the mechanisms of vortex formation. For application, it will be essential to understand the degree to which these vortices can be engineered. For example, is it possible to manufacture a regular array of vortices by defect engineering in thin Fe2O3 samples, a useful geometry for a magnetic memory device.